SHORT-RANGE WEATHER FORECASTING 
upper-air analysis. Radiosonde observations provide 
pressure, temperature, and moisture data and values 
of these elements at significant points on the ascent 
curve are plotted on various thermodynamic diagrams. 
In the United States, the pseudoadiabatic diagram is 
commonly used. 
Constant-Pressure Charts. To provide a horizontal 
picture over a given area, the same information is 
plotted for certain standard isobaric surfaces. At most 
district forecast centers, these usually include the 850-, 
700-, 500-, and occasionally the 300- and/or 200-mb 
surfaces. Weather centrals will usually also prepare the 
1000- and 100-mb charts, differential analysis charts or 
chart and a maximum potential-temperature chart. 
Pressure Charts. Pressure and the 3-hr pressure 
change and characteristic are entered on the pressure 
chart directly from the coded observation, and the 12- 
hr pressure change, corrected for the diurnal variation 
in pressure, is computed and plotted. Three and 12-hr 
isallobars are drawn and maxima and minima are 
tracked. 
Temperature Charts. Charts contaming current and 
maximum or minimum temperatures are usually plotted 
at 1200Z and 2400Z and the 24-hr change from maxi- 
mum to maximum, or minimum to minimum as the 
case may be, is computed. Departures from the normal 
temperature are usually entered. 
Other Charts. Graphs of the zonal index and mis- 
cellaneous charts, such as snow-on-the-ground and the 
24-hr precipitation, are prepared for various purposes. 
USE OF SURFACE DATA 
Basic Surface Chart. When entry of data on the sur- 
face chart has been completed, the analyst locates and 
defines the fronts, draws the isobars, and determines 
the air masses. Procedures in general use are described 
by Petterssen [42, Chaps. I and XI]. The current rate 
of movement, and the acceleration and deceleration 
tendencies of fronts and pressure systems are deter- 
mined, and the previous paths of high and low centers 
are plotted. Local and district forecast offices use 
weather-central analyses but district offices make such 
modifications as their denser network of observations, 
special reports, and pilot reports may indicate. Areas 
of hydrometeors are indicated in accordance with uni- 
form national custom or regulation. Conclusions on the 
causes of current and past weather, drawn from sur- 
face maps, are integrated and reconciled with those 
derived from the upper-air and auxiliary charts. 
Pressure-Change and Other Auxiliary Charts. The 
necessity for the pressure-change chart arises from the 
large amount of data, particularly im bad weather, en- 
tered on the surface chart for each individual station. 
The severe deadlines imposed on analysts and fore- 
casters im many weather services preclude close in- 
spection and analysis of these data at each station. The 
pressure chart presents in clearly defined form 3- and 
12-hr pressure trends. In summer, the true 3-hr pressure 
change is masked by the greater diurnal change, and 
the 12-hr change, corrected for the diurnal variation in 
pressure, is more representative. 
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Cook [16] has collected some hundred empirical rules 
for use of the pressure-change chart, many, however, 
of only regional value. The 12-hr pressure change rep- 
resents the net change in surface pressure in that period 
of time resulting from all the physical processes which 
affect the weight of a column of air extending from the 
surface to the top of the atmosphere. The direction and 
rate of movement and changes in the intensity of the 
allobars can be forecast with the same accuracy as the 
highs and lows themselves and the relationship is ob- 
vious. 
Whether the various areas of ecyclogenesis and anti- 
cyclogenesis are undergoing intensification or weakening 
can be deduced rather quickly and accurately from the 
pressure-change chart by comparing the last 3- and 12- 
hr pressure changes with the same allobaric values 6 and 
12 hr before and for other time intervals. The explosive 
effect as a surface pressure fall reaches an area under a 
cold upper-air low or when the leading edge of a new 
pressure fall reaches a moist stationary front is well 
known to all forecasters. Miller [85] has noted that 
cyclogenesis can be recognized on the Atlantic Coast by 
12-hr pressure changes when 3-hr changes are indistinct. 
Cyclogenesis on the east slope of the Rockies can be 
handled much better on the basis of 3- and 12-hr katallo- 
bars than by actually following pressure systems as they 
move out of the Gulf of Alaska. 
A snow-on-the-ground chart is necessary for fore- 
casting the rate of modification of air masses as they 
move from snow cover to bare ground or vice versa and 
for the forecasting of maximum and minimum temper- 
atures and drifting snow. 
Some forecast centers maintain a graph of the zonal 
index, plotting daily values or using mean values pro- 
vided by the extended-forecast section. The zonal in- 
dices provide a good indication of the broad-scale fea- 
tures of the general circulation and assist in forecasting 
changes in intensity or position of the centers of ac- 
tion. Attempts to correlate variations of the zonal index 
with the formation of arctic air masses have, so far, 
been unsuccessful and attempts such as that of Weiss 
[57] to determine the relationship, suggested by Rossby 
[47], between a large-scale flow of warm air northward 
in the eastern Pacific Ocean toward Alaska and marked 
anticyclogenesis over the Mackenzie Basin have gen- 
erally failed. In this particular situation, anticyclo- 
genesis appears to take place at more southern lati- 
tudes. Namias [36] has described a case where a general 
fall in index over Asia was followed by the progressive 
establishment of low-index conditions from west to east 
resulting in strong polar anticyclogenesis. It has been 
known for some time that an analogous condition may 
be initiated in Europe, developing westward into North 
America and resulting in the condition generally re- 
ferred to as ‘“‘blocking.”’ Indeed, exceptionally strong 
and persistent arctic anticyclogenesis seems to form 
more often in the latter than in the former manner. 
These important trends are not often available to the 
district forecaster but fairly short-term forecast effects 
arise from them since it is not unusual for an arctic 
